The present invention relates to fracturing fluids and to a method for delivering a material into a subterranean formation and, more particularly, to a method for breaking a fracturing fluid using a controlled release capsule for delivering the breaker to the fracturing fluid in the subterranean formation and the fracturing fluids containing such controlled release capsule.
In the drilling of a well and the subsequent recovery of fluids such as crude oil and natural gas from subterranean formations, various materials are added to the well bore and/or subterranean formation to improve the efficiency of the well drilling operation, to increase production of the fluids from the formation and/or to terminate and seal a non-producing well. These treating agents are generally added, in their active form, above ground and flowed into the well bore or through the well bore to the subterranean formation.
For example, a subterranean formation is often subjected to a fracturing treatment to stimulate the recovery of fluids such as crude oil or natural gas. In fracturing the formation, a fluid is introduced into the well at a rate and pressure sufficient to produce one or more fractures in the formation and/or enlarge or extend existing fractures. The fluid can contain a propping agent such as sand or sintered bauxite which is deposited in the fractures to maintain the fractures and/or an acid which reacts with the acid-soluble components of the formation thereby creating and enlarging passageways for the flow of fluids from the formation.
Heretofore, in both hydraulic fracturing and fracture acidizing operations, a high viscosity, gelled aqueous fluid, a high viscosity water/hydrocarbon emulsion or a foam of a gas/water dispersion has typically been employed. These high viscosity fluids are capable of the necessary penetration to realize maximum benefits from the fracturing operation and in suspending the propping agents, if employed, without excessive settling. In addition, the high viscosity fluids are capable in opening the fractures to a greater width than is possible with the low viscosity fluid.
After the high viscosity aqueous solution or emulsion has been pumped into a subterranean formation and the fractures formed, it is generally desirable to convert the gel or emulsion into a low viscosity fluid, thereby allowing the fluid to be removed from the formation and the desired material such as oil or natural gas, to flow back through the fractures into the wellbore. This reduction of the viscosity of the fracturing fluid is commonly referred to as "breaking" the gel or emulsion. Conventionally, this breaking of the gel or emulsion is carried out by adding a viscosity reducing agent (commonly called a "breaker") to the formation at the desired time. Unfortunately, the described techniques often result in insufficient breaking (i.e., an insufficient reduction in the viscosity of the fracturing fluid) or premature breaking of the fluid. Premature breaking of the fluid causes a reduction in the viscosity of the fracturing fluid prior to the desired termination of the fracturing operation, thereby reducing the overall effectiveness of the operation.
One proposed method for controlling and/or delaying the activity of the viscosity reducing agent or "breaker" consists of introducing to the subterranean formation the breaker in a prill or pellet formed by combining the breaker with a hydratable gelling agent and gell degrading substance and forming the resulting mixture into the desired prill or pellet form. See, for example, U.S. Pat. No. 4,202,795. Upon exposure of the prills or pellets to an aqueous fluid, the gelling agent is said to hydrate and form a protective gel around each of the pellets, thereby preventing the release of the breaker into the aqueous fluid until the protective gel is broken by the gel-degrading substance. In this manner, the cited reference claims that the breaker can be released to the aqueous fluid in a controlled manner. It appears that a relatively large amount of the hydratable gelling agent is required to prepare the pellets and that the amounts of the hydratable gelling agent must be carefully controlled. In addition, the time period over which the pellets are released may vary substantially.
In another method for delaying the release of a breaker, U.S. Pat. No. 4,506,734 proposes introducing a viscosity reducing chemical contained within hollow or porous, crushable beads in the hydraulic fracturing fluid. The viscosity reducing agent is said to be released upon the crushing of the beads which results from the closing of the fractures caused by the fracturing fluid passing or leaking off into the formation or by removing the fluid by backflowing. Unfortunately, the stresses caused by the closing of the formation will affect the percentage of beads being crushed so that a large percentage of beads may remain unbroken, particularly if the formation closes at a slow rate. In addition, a large percentage of the beads may be crushed in one area of the formation being treated, whereas in a second area of the formation, a substantially lower amount of the beads may be crushed. Therefore, inconsistent performance can be expected.
Although the foregoing methods appear to provide for delayed release of a breaker into the subterranean formation, it remains desirable to provide an alternative method and commercially more practicable method which give equivalent or superior performance.